The present invention relates to a method for the automatic positioning of a ship, particularly a drilling ship, by which the ship can be moved, by propulsion means, in the direction of the main external disturbance, or position deviation, forces.
In order to perform the so-called dynamic positioning of a ship there must be provided propulsion systems which act to hold the ship accurately at a working position. Examples of the type of ship involved are measuring ships, research ships and drilling ships. The propulsion systems are drive propellers with pivotal slipstream (e.g. Voigt-Schneider, Schottel) or adjustable propellers. Such systems are presently being produced only up to medium power capacities. To meet higher power requirements, transverse thrust arrangements may be provided. It is also possible to combine transverse thrust systems with active propellers.
Since transverse thrust systems possess a lower total system efficiency than do active propellers for certain directions of the external disturbance forces, they must be installed right from the start to be capable of satisfying higher power requirements.
When a ship is being dynamically positioned, the ship is preferably placed with its bow facing in the direction of the resultant of the disturbance forces since in this position it will have the lowest wind, water and wave resistance, i.e. the external influences will apply the lowest force levels to the ship.
Disturbance forces which attack a ship from an oblique direction even further reduce the efficiency of transverse thrusters since, due to the technical design concepts involved, such systems cannot be adjusted in the direction of the effective disturbance forces but must compensate the forces by providing thrust components in the longitudinal and transverse directions, the transverse direction being at right angles to the longitudinal axis of the ship.
Methods which attempt to regulate the disturbance torque acting on a ship so that it becomes zero cannot be successful, for example if the ship has an asymmetrical superstructure, which is usually the case for the above-mentioned special types of ships. A further drawback of this process is that the curve representing the disturbance torque as a function of ship heading direction has two minima within an angular range of .pi./2, so that when there is a sudden change in the direction of the resultant disturbance force, for example due to a shift in the wind, unstable states may develop.